This research is designed to obtain detailed information on the mechanisms by which lactogens (prolactin and placental lactogen) and placenta are involved in regulation of lactogen receptor in uterus and ovary during pregnancy. This information will be useful in evaluating the role of lactogens and lactogen receptor in maintenance of pregnancy and fetal development. The approach to this problem involves simulation of pregnancy by means of pseudopregnancy induction in rabbits and maintenance of this pseudopregnancy by injection of placental extracts and progesterone which would allow evaluation of lactogen receptor under semicontrolled conditions. A second major focus of this research will be to carry out detailed kinetics of the reaction of lactogen receptor with bovine prolactin and ovine placental lactogen. Lactogen receptor will be characterized kinetically by both fluoroligand and radioligand methods during early, late and simulated pregnancy. The third major area of focus of this project will be efforts toward purification of the lactogen receptor in uterus. Endometrium has been determined to be a source of high specific binding activity of lactogens in mid-pregnancy. Purification of this receptor will involve preparation of plasma membrane fragments, solubilization of these fragments and chromatography by conventional and affinity methods. Receptor prepared in this project will be characterized kinetically, chemically and physically.